1. Field of the Invention
This invention relates generally to well production equipment, and in particular to a pneumatic submersible pump.
2. Description of the Prior Art
Various types of artificial lifts are used to bring liquids to the surface of the earth when the pressure of the liquid-bearing reservoir is insufficient to produce the formation fluids by natural means. The pumping motion of the artificial lift may originate at the surface of the ground, or below, as a result of the application of electrical or fluid power to a subsurface pump. In the usual surface powered rig, a vertically reciprocating pump element at the bottom of the well is actuated by a walking beam pivotally mounted on a Sampson post and connected at one end to a sucker rod string and at the other end to a prime mover which supplies power through a Pitman gear for producing the reciprocating motion of the sucker rod string.
Generally, in the surface powered rig, the prime mover consists of an internal combustion engine or electric motor. The cost of this prime mover, as well as its operation and maintenance, is, in many instances, a significant economic factor in the production of liquids from subterranean liquid-bearing reservoirs. The sucker rods are characterized by a short, fast stroke, resulting in low pump efficiency, high power consumption and low recovery rates. The short, fast stroke results in a churning action which causes the formation of emulsion.
An additional limitation of surface-powered rigs which operate a sucker rod through rigid production tubing is that, for practical purposes, the weight of the sucker rod for wells having a producing formation at about 10,000 feet or deeper is excessive, resulting in stretching and early failure of the rods. Moreover, the sucker rod assembly is subject to severe wear in slant-hole or crooked-hole wells. In slant-hole wells, of the type typically drilled offshore, the sucker rod is subjected to severe frictional wear because of the slant of the hole, and is therefore subject to early failure and requires frequent replacement. Frictional wear is also a serious problem in crooked holes in which the well bore follows a helical path. Because of the difficulty of drilling a vertical well at other than shallow depth, effective use of the sucker rod pumping assembly is substantially limited to shallow wells.
The cost of the prime mover, as well as its operation and maintenance, is in many instances a significant economic factor in well production operations which utilize sucker rods. Moreover, the sucker rod pump is characterized by short, fast stroke action which is not compatible with the slow supply rate of weak natural formations. It will be appreciated that the substantial captial expenditure associated with the launching, recovering, repair and operation of sucker rod units makes its use prohibitive in low production wells, and accounts for a substantial percentage of the overall production costs for other wells.
For the foregoing reasons, there has been considerable interest in improving pumping systems in which the motive force is provided by fluid power applied to a subsurface pump, thereby eliminating the sucker rods and affording precise control of the pumping action.